When you click on the "edit" button it change your view to something that looks a lot like a regular text editor with all the normal options to try out
When using the "link" button you can make links to both pages in the wiki you create and "external" links to cool stuff you want to show your kids. Notice in the example below you can make the text different from simply typing the full address out for the kids
If you want to add a "file" you simply follow the instructions to add whatever image, document or ppt you want the kids to have access to. Images and documents show up like the examples below. Notice that you can "embed" the file where it's visible right on the page or link to it where it requires a click.
"Widgets" are pretty sweet and there's lots to play with here. Wikispaces set them up so it's quite self-explanatory. Below is an example of a video widget. Just follow the instructions for whatever type of content you're looking to add. Note that in general a widget points to content already on the web, so that if what you want isn't already on another site (like a slideshow, calendar, audio file) you can't make it a widget:(
How to Use the Edit page feature